Should I replace these valve shims?

[patrickhimself]

In my continuing quest to track down what's keeping the fire out of my #4 cylinder, I just rechecked all the valve clearances on my 1979 GS750L, as I realized I had been doing it "wrong" before. All of the valves are within specs (much to my chagrin, really), except the #2 and #3 exhaust shims, which are both somewhere between .10mm and .15mm (I don't have a feeler in between). So I'm wondering, since I'm in there, should I swap out those shims to close up that clearance a bit?

[tkent02]

except the #2 and #3 exhaust shims, which are both somewhere between .10mm and .15mm (I don't have a feeler in between)

Sure you should, but that's not what is keeping number 4 from firing.

Spark, compression, fuel?

[patrickhimself]

Sure you should, but that's not what is keeping number 4 from firing.

Spark, compression, fuel?

Yeah, still trying to track that down.

Spark is good. Plugs are getting wet. Compression is consistent at WOT, but it's about 20psi below the others with the throttle closed (I know that's not a scientific test, necessarily, but it seems odd). I'm thinking the next step is checking if something's up up with the pistons/rings.

I'll get on the shims.

[tkent02]

I'm thinking the next step is checking if something's up up with the pistons/rings.

Last resort, or at least it should be. What's the history of the bike? Sat for years or a daily driver?

How long since it's run correctly?

Spark is good, but is the spark happening while the plug is in the cylinder? Sometimes they spark out in the air but not in the motor. Also is the spark happening at the right time?

Could also be too much fuel on that cylinder, too rich and it won't be able to fire at all.

[patrickhimself]

Bought the bike last year. Was a daily rider until about November, when I decided it was a good idea to start doing "routine" maintenance (it was raining...). Hasn't run since. Well, it runs, but cylinder #4 is cold.

Dumb question: how does one check for spark in the cylinder?

[rwnielsen]

Each coil does two cylinders. Switch those wires and see if the problem follows the wire

Just an idea

[mcycle-nut]

Dumb question: how does one check for spark in the cylinder?

Either that or pull the plug cap, put a spark plug in it, ground the plug on an unpainted part of the motor, and crank over the motor. You should she a spark.

[tkent02]

Put a different spark plug in the one you are concerned with. Swap with a cylinder that's running fine. The not sparking under compression is rare, but it can happen.

Are you saying the #4 never fires at all, as in the exhaust pipe stays cold, even when you ride the bike?

Or just sitting there idling #4 isn't firing?

[wymple]

In my continuing quest to track down what's keeping the fire out of my #4 cylinder, I just rechecked all the valve clearances on my 1979 GS750L, as I realized I had been doing it "wrong" before. All of the valves are within specs (much to my chagrin, really), except the #2 and #3 exhaust shims, which are both somewhere between .10mm and .15mm (I don't have a feeler in between). So I'm wondering, since I'm in there, should I swap out those shims to close up that clearance a bit?

.10-.15 mm is .004-.006 thousandths. Why would you close that down?

[tkent02]

Because it's too big. The spec is .03 - .08mm, most of us will go to .10 but any bigger is not good.

[patrickhimself]

Put a different spark plug in the one you are concerned with. Swap with a cylinder that's running fine. The not sparking under compression is rare, but it can happen.

Are you saying the #4 never fires at all, as in the exhaust pipe stays cold, even when you ride the bike?

Or just sitting there idling #4 isn't firing?

Sitting and idling #4 isn't firing. If I take it riding, or even just crank the revs up above 3k, it'll spit out some black smoke and start warming up. To me this says something is wrong in the carb idle circuit, but I've cleaned (full dip) and bench synched and vacuum synched and such multiple times.

I've tried swapping the plugs and plug wires around several times. Doesn't seem to change anything.

A little more bike history:

PO said the bike had been sitting in his son in law's garage for 10+ years. He got it running, but there are a few shortcuts and dumb mods on her for sure. He installed a solid state ignition, so I think the timing should be fine(?).

Anyway, I'll get on those valves.

[tkent02]

If I take it riding, or even just crank the revs up above 3k, it'll spit out some black smoke and start warming up. To me this says something is wrong in the carb idle circuit,

Sure sounds like it.